Sony pushed out its slightly faster PS4 Pro a few months ago, and with the new v4.5 firmware, the unannounced "Boost Mode" was included.
The new "Boost Mode" promised "improved gameplay, higher frame rates, for some games that were released before the introduction of the PS4 Pro". Fast forward to now, the fine folks at Digital Foundry have used the PS4 Pro's Boost Mode, reporting that some games enjoy a massive 38% performance increase.
The increase in performance depends on the game, but with Destiny used as an example - there is no performance benefit, as the game is locked to 30FPS. But in something like Battlefield 1, it can "stay at a solid 60FPS in Boost Mode, without the frequent frame-rate dips that can occur during a 64-player match in base mode".
Most games are seeing a performance boost of 14% on average, but that works out perfectly with the 14% faster clock speed on the GPU inside of the PS4 Pro over the original PS4 console - from 800MHz, to 911MHz. The additional 18 compute cores inside of the GPU on the PS4 Pro don't seem to helping out performance wise, as there needs to be game-specific changes made by the developer.
Assassin's Creed Unity goes from 22-24FPS average without Boost Mode, up to 30FPS consistently. Project CARS has a much bigger 35-38% increase courtesy of Boost Mode.